Detoxification, active uptake, and intracellular accumulation of chromium species by a methane-oxidizing bacterium.

Despite the wide-ranging proscription of hexavalent chromium, chromium (VI) remains among the major polluting heavy metals worldwide. Aerobic methane-oxidizing bacteria are widespread environmental microorganisms that can perform diverse reactions using methane as the feedstock. The methanotroph Methylococcus capsulatus Bath, like many other microorganisms, detoxifies chromium (VI) by reduction to chromium (III). Here, the interaction of chromium species with M. capsulatus Bath was examined in detail by using a range of techniques. Cell fractionation and HPLC-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) indicated that externally provided chromium (VI) underwent reduction, and was then taken up into the cytoplasmic and membranous fractions of the cells. This was confirmed by X-ray photoelectron spectroscopy (XPS) of intact cultures that indicated negligible chromium on the surface of, or outside, the cells. Distribution of chromium and other elements within intact and sectioned cells, as observed via transmission electron microscope (TEM) combined with energy-dispersive X-ray spectroscopy (EDX), and electron energy loss spectroscopy (EELS), was consistent with the cytoplasm/membrane location of the chromium (III), possibly as chromium phosphate. The cells could also take up chromium (III) directly from the medium in a metabolically dependent fashion, and accumulate it within the cells. These results indicate a novel pattern of interaction with chromium species distinct from that observed previously with other microorganisms. They also suggest that M. capsulatus, and similar methanotrophs may contribute directly to chromium (VI) reduction, and accumulation in mixed communities of microorganisms that are able to perform methane-driven remediation of chromium (VI).ImportanceM. capsulatus Bath is a well characterised aerobic methane-oxidising bacterium that has become a model system for biotechnological development of methanotrophs to perform useful reactions for environmental clean-up, and making valuable chemicals and biological products using methane gas. Interest in such technology has increased recently owing to increasing availability of low-cost methane from fossil, and biological sources. Here, it is demonstrated that the ability of this versatile methanotroph to reduce the toxic contaminating heavy metal chromium (VI) to less toxic chromium (III) form occurs at the same time as accumulation of the chromium (III) within the cells. This is expected to diminish the bioavailability of the chromium, and make it less likely to be re-oxidised to the toxic chromium (VI). Thus M. capsulatus has the capacity to perform methane-driven remediation of chromium-contaminated water, and other materials, and to accumulate the chromium in the low-toxicity chromium (III) form within the cells.